1. Field of the Invention
The present invention relates to a developing device using a two-ingredient type developer and a copier, printer facsimile apparatus or similar monochromatic or color image forming apparatus.
2. Description of the Background Art
It is a common practice with an image forming apparatus to form a latent image on an image carrier, develop the latent image with a developing device to thereby produce a corresponding toner image, transfer the toner image to a sheet or cording medium, and fix the toner image on the sheet. The developing device, in many cases, uses a two-ingredient type developer made up of nonmagnetic toner grains and magnetic carrier grains. The developer is scooped up onto a developer carrier and caused to form a magnet brush thereon. The magnet brush is brought into contact with the image carrier, so that the toner grains deposit on the latent image formed on the image carrier.
The developer carrier includes a rotatable sleeve and a plurality of magnets fixed in place inside the sleeve. One of the magnets forms a main pole for development in the developing zone of the surface of the sleeve that faces the image carrier. In the developing zone, the carrier grains included in the developer rise in the form of brush chains along the magnetic lines of force of the main pole, thereby forming the magnet brush. This kind of developing system is generally referred to as a contact, two-ingredient type developing system. Although this type of developing system needs sophisticated control over the toner content of the developer and is bulky, it is predominant over the other developing systems because of high image quality and maintainability achievable therewith.
To cause the toner grains to move from the developer carrier to the image carrier, the contact, two-ingredient type developing system forms an electric field for causing the toner grains to leave the magnet brush approached the image carrier and deposit on the latent image. More specifically, the toner grains leave the carrier grains in cloud- or smoke-like groups due to the behavior of the carrier grains. The groups of toner grains are caused to move toward the latent image by the electric field. To promote the efficient movement of the toner grains, the peak of the magnetic lines of force of the main pole is located at a position where the developer carrier and image carrier are closest to each other, so that the highest portion of the magnet brush coincides with the developing zone.
In practice, however, the ratio by which the toner grains released from the magnet brush are used (efficiency) achievable with the contact, two-ingredient type developer is presumably low. In light of this, Japanese Patent No. 3,015,116, for example, discloses a developing system in which toner grains are deposited on a developer carrier in the form of a thin layer and then applied with an AC bias via, e.g., a wire electrode in a developing zone to thereby form a toner cloud. Japanese Patent Nos. 3,023,999, 3,077,235, 3,084,465, 2,850,504 and 2,668,781, Japanese Patent Publication No. 8-44214 and Japanese Patent Laid-Open Publication No. 8-44214 also propose various schemes for promoting the efficient use of toner grains. The schemes proposed in these documents are identical with the above Patent No. 3,015,116 in that a new arrangement and a power supply are added to the basic construction of the contact, two-ingredient type developing system.
Further, to promote efficient development, a plurality of developer carriers may be arranged, as taught in Japanese Patent Laid-Open Publication Nos. 2-173684 and 8-278691 by way of example.
A problem with the AC bias scheme using, e.g., a wire electrode is that it needs an exclusive arrangement and an exclusive power supply for producing more toner clouds in addition to the basic construction of the developing system, resulting in a sophisticated configuration and an increase in power consumption. In addition, it is difficult to form a thin toner layer on the developer carrier. Another problem is that irregularity appears in an image due to the contamination of the wire electrode. This is true not only with a wire electrode but also with any other implementation for producing more toner clouds.
The system using a plurality of developer carriers is undesirable because it increases the overall size and cost of the image forming apparatus.
When a distance between the developer carrier and the image carrier, i.e., a development gap is increased, the force with which the magnet brush rubs the latent image decreases and reduces the omission of the trailing edge of an image while promoting faithful reproduction of horizontal lines. However, such a development gap causes the toner grains to deposit on the edges of the latent image in a large amount (so-called edge effect or edge enhancement). More specifically, the edge effect renders solitary dots larger than expected, thickens lines, enhances the contour of a solid portion or that of a halftone portion or omits the outside of such a portion. The edge effect therefore makes control over tonality reproduction sophisticated.
Although a small development gap reduces the edge effect and protects images from granularity, it intensifies the force with which the magnet brush rubs the image carrier. This, coupled with the influence of the charge of opposite polarity deposited on the carrier grains, brings about the omission of the trailing edge of an image and unfaithful reproduction of horizontal lines and dots, resulting in a direction-dependent image.
On the other hand, Japanese Patent Laid-Open Publication No. 5-303284 teaches a non-contact type developing system in which two magnetic poles sandwich a developing zone in the vicinity of an image carrier while a gap between a developer carrier and the image carrier is sized greater than the thickness of a developer layer. In this configuration, the developer is caused to jump up from the developer carrier. With such a developing system, it is possible to extremely faithfully reproduce a highlight portion and implement a high-definition halftone portion. However, the development efficiency available with this developing system is low and likely to bring about short density and blur of a black solid portion.
I proposed a new developing system, which is not known in the art, including a developer carrier facing an image carrier and accommodating magnets therein and causing a two-ingredient type developer to deposit on the developer carrier in the form of a layer. A difference in speed is provided between the developer carrier and the magnets in order to cause the developer layer to flow at least in a region where the developer carrier and image carrier face each other, while forming a magnet brush. During the flow, free toner grains released from magnetic carrier grains are caused to deposit on a latent image formed on the image carrier.
It was experimentally found that the developing system using the free toner grains was advantageous over the magnet brush type developing systems effecting development only in the region where the carrier grains contact the image carrier in the following aspect. The developing zone is extended because of the free toner grains available for development, so that the amount of development and therefore development efficiency is increased. This insures a solid image portion having high density.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 5-303284, 2000-305360 and 2001-51509.
It is an object of the present invention to provide a developing device capable of efficiently using toner while solving the problems discussed above, and an image forming apparatus using the same.
It is another object of the present invention to provide a developing device capable of obviating granularity and the omission of the trailing edge of an image (including image noise, e.g., unfaithful reproduction of horizontal lines and omission of dots) that are dependent on a development gap in a tradeoff relation, and an image forming apparatus using the same.
It is a further object the present invention to provide a developing device capable of further enhancing efficient development to thereby provide even a black solid image with high density, and an image forming apparatus using the same.
A developing device of the present invention includes a developer carrier accommodating stationary magnetic field generating means there inside for scooping up a developer, which is made up of non-magnetic toner grains and magnetic carrier grains, onto the developer carrier to thereby form a magnet brush. The magnet brush is caused to contact the image carrier to thereby develop a latent image formed on the image carrier. The carrier grains forming the magnet brush are disturbed in a developing zone.
A least two brush chain forming portions where the magnet brush rises may be formed in a region where the developer carrier and image carrier face each other.